Proper soil temperature is critical to insure seed germination in a garden bed. Seeds of most garden plants require a minimum nighttime temperature of about 68 degrees Fahrenheit for at least three to fourteen and up to twenty-one days to germinate. Commercial growers have moved their farms to the South and West in order to better guarantee favorable soil temperatures during the critical germination period. Backyard gardeners throughout the rest of the country have several options. They may plant seed and hope for proper soil temperatures, buy "started" plants for transplanting, or use expensive and cumbersome greenhouses and window boxes to create proper germination conditions. Seeds will not germinate when soil is not maintained at the critical temperature and may even rot if planted in soil which is too cool.
Soil temperature also is important during the late stages of the growing season. The leaves and unripened fruit of garden plants can be protected from frost damage by plastic `tents`, but the plants will not continue to produce when the soil temperature drops below a critical temperature. Thus, devices which warm garden soil can increase plant production by extending the growing season into the autumn months when soil temperature normally drops too low.
Raised garden beds have previously been constructed from boards or heavy logs nailed together or railroad ties. These beds are difficult to assemble and disassemble from year to year and thus, when left in place, require the gardener to prepare the soil by hand. Raised beds are desirable, however, because they exhibit good drainage and do not require the gardener to bend over as far when tending the plants.
Cold frames have been used for many years to protect young plants from damaging spring frosts. Cold frames usually consist of a low-lying frame of wooden boards and a glass cover. Although cold frames insure warm daytime temperatures, they often do not significantly insulate the soil to prevent the nighttime soil temperature from dropping below the critical seed germination temperature. U.S. Pat. No. 4,429,489 (Fischer) discloses a cold frame including sidewall panels and a cover. The sidewall panels are hollow and serve to insulate the garden bed from temperature extremes, but do not collect and store solar energy.
The hot bed is another device which is often used to enhance early season growth of garden plants. Hot beds include side panels, a glass roof and an internal source of heat such as fermenting manure. If the fermentation reaction or other source of heat fails, seeds planted in the hot bed may not be sufficiently warmed and may fail to germinate. Other structures and devices which are directed to the maintenance of proper growing environments include a greenhouse having hot water pipes for warming soil (U.S. Pat. No. 4,309,843); a greenhouse having a passive solar heating system (U.S. Pat. No. 4,244,148); insulated plant containers (U.S. Pat. Nos. 3,158,957; 4,395,486; and 4,016,676); and a device comprising a pool of water in which plant pots are floated (U.S. Pat. No. 4,041,641).